Direct current electromotor



Oct. 6, 1931. F. BOHM 1,826,342

DIRECT CURRENT ELEGTROMOTOR Filed March 8, 1929 Iatented Oct. 6, 1931 UNITED STATES FRIEDRICH 3611M, OF STUTTGART, GERMANY DIRECT CURRENT ELECTROMOTOB Application filed March 8, 1929, Serial No. 345,465, and in Germany December 29, 1928.

The object of this invention is to increase the range within which the speed of direct current electromotors can be regulated,

while maintaining a good degree of efficiency I at all speeds, and to increase also the pre-,

cision with which the regulation is effected.

To this end I use in combination with the usual pair of brushes, or with each of the pairs, where there are more than one, an

' auxiliary brush and an auxiliary resistance supplementary to the usual starting resistance, the auxiliary resistance being connected to the auxiliary brush, and I arrange that when starting the motor the auxiliary resist- 3'7 ance is progressively put into circuit as the starting resistance is cut out, the connection oi the auxiliary resistance to the line being then broken, so that the auxiliary brush is dead at or about the time when the normal 2' starting resistance is fully cut out.

The eiiectiveness of this arrangement, for producing a wide range and great precision of regulation, is increased by using a starting resistance having a large number of subdivisions, and a commutator having more than the usual number of segments or bars, and also by having a very small air gap between the armature and the field magnet poles. For the windings I prefer to use wire insulated by enamelling, which saves space and thus increases the compactness of the machine.

An embodiment of the invention is shown in the accompanying drawings, in which Fig. 1 is a longitudinal section of the electromotor, and,

Fig. 2 a cross section thereof, Fig. 3 being a circuit diagram. The armature of the motor has a commutator a with the usual brushes 6, and a field magnet c. In the embodiment shown there are two brushes 1), which in Fig. 3 are marked b and (2 respectively, but it will be understood that there may be more than one pair of these brushes, as is frequently the case in direct current motors. In addition to the brushes 6 there is an auxiliary brush 0, set at an angle a of about to the positive brush +6. If there were two pairs of brushes 6 there would be an additional auxiliary brush a.

The controller includes two resistances d,

- f, and an arm h, the resistance 03 being in the armature circuit, and the resistance f being in the field circuit. In addition there is a resistance g 9 connected to the brush 0. The rheostat arm it has three contacts, one for each of the resistances d, f and g 9 For starting the motor the arm h is moved to position I (Fig. 3). In this position the 50 whole of the resistance f is cut out of the field circuit, the whole of the resistance (1 is in the armature circuit between the brushes b and b, and the small part g of the resistance 9 g is in the external part of the armature circuit between the brushes 0 and +1). The motor may then run at a speed of, say 400 revolutions per minute. By rotating the arm h clockwise the resistance (Z is gradually cut out of circuit, and the part g of resistance g g is gradually put into circuit. When the arm reaches position II it has cleared the resistance 9, and cut all the resistance of out of circuit. The brush 0 is then dead This is the normal position for the rheostat arm h, the motor then running at its normal speed,- say 800 revolutions per minute. To

increase this speed, it required, the arm is moved farther towards position III, in order to weaken the field by putting resistance f into the field circuit. With the brush in position III the speed may be 3200 revolutions per minute.

I prefer to use a commutator a with a much larger number of bars than is usual. Where normally 24 bars would be used I use, say, from 48 to 60 bars, an increase of from to I prefer also to use a very small air gap 2' (Fig. 1) between the armature and pole pieces, say a gap of .25 mm., grinding the .9

pole faces and the teeth of the armature to obtain the accurate circular shape required with this small gap. Viewed axially, the length of the windings on the field magnet and armature is about 30% larger than according to current practice. I prefer to use ball or roller bearings for the armature shaft, designing the rotor so that it is statically and dynamically balanced, to run practically without vibration.

With the novel construction described very great advantage is obtainable from using high grade materials for the casing, fieldmagnets and armature, and from precision inmanufacture. The invention is well suited for motor of upwards of A H. P. and the motor can be made very compact, say an overall length of 312 mm. and diameter of 180 mm. for amachine' of 1 P.,- this machine consuming. about 840 watts, and working with practically the same efficiency at 400 R. P. M. as at 3200B. P. M. A rangeof speed greater than 1: S, and amounting to as much as 1:12, is obtainable with heavier windings. Extensive subdivision of the resistances, using up to about 30 contact studs enables the speed to be regulated with a precision practically equal to that obtainable with hydraulic gear.

lVhat I claim as my invention and desire to secure by Letters Patent of the United States is I 1. In a direct current electronrotor the'combination, with an armature circuit including a commutator, a positive and a negative brush on said commutator'and a main starting resistance, of a third brush on said commutator, an auxiliary starting resistance connected to said third brush, and means for gradually cutting said main resistance out of said armature circuit and simultaneously putting sai'd auxiliary resistance gradually into circuit in series with said third brush and one of said other brushes.

2. Ina direct current electromotor the combination, with an armature circuit including commutator, a positive and a negative brush on said commutator and a mainstarting resistance, of a third brush on said commutator, an auxiliary starting resistance connected to said third brush, and means for gradually cutting said main resistance out of said armature circuit and simultaneously first putting said auxiliary resistance gradually into said circuit and then disconnecting. said auxiliary resistance from the line.

3. The combination claimed in claim 1, the third brush being a negative brush set at an angle of about 55 to the positivebrush.

4:. In a direct current electromotorthe combination, with a commutator and two brushes on said commutator, of a regulatable starting resistance connected to one of said brushes, a controller arm for gradually cutting s'a'i'd starting resistance out of circuit, a third brush on said commutator, and a regulatable auxiliary resistance which is connected to said third brush and is increasingly put into circuit by the movement of the controller arm which gradually cuts said starting resistance out of circuit.

5. In a direct current electromotor the combination, with a commutator and two brushes on said commutator, of a regulatable starting resistance connected to one of said brushes, a controller arm for gradually cut-ting said starting resistance out of circuit, a third brush on said commutator, and a regulatable auxiliary resistance which is connected to said third brush and is increasingly put into circuit by the movement of the controller arm whichgradually cuts said starting resistance out of circuit, a portion of said auxiliary resistance being in circuit whenv said arm is in the position leaving the whole of said starting resistance in circuit.

In witness whereof Ihave signedthis specification.

FRIEDRICH BOHM. 

